Tests: prefer to use first rather than indexing

In the case of a failure in the test, we would previously continue to
perform the slice operation which will fail in the case of an Asserts
runtime due to an out of bounds access.  The use of `first?` here would
unwrap or fail, allowing tests to continue execution.

Author: compnerd

Tests/SPMBuildCoreTests/PluginInvocationTests.swift

@@ -299,7 +299,7 @@ class PluginInvocationTests: XCTestCase {
             XCTAssert(packageGraph.packages.count == 1, "\(packageGraph.packages)")
 
             // Find the build tool plugin.
-            let buildToolPlugin = try XCTUnwrap(packageGraph.packages[0].targets.map(\.underlyingTarget).first{ $0.name == "MyPlugin" } as? PluginTarget)
+            let buildToolPlugin = try XCTUnwrap(packageGraph.packages.first?.targets.map(\.underlyingTarget).first{ $0.name == "MyPlugin" } as? PluginTarget)
             XCTAssertEqual(buildToolPlugin.name, "MyPlugin")
             XCTAssertEqual(buildToolPlugin.capability, .buildTool)
 
@@ -873,7 +873,7 @@ class PluginInvocationTests: XCTestCase {
             XCTAssert(packageGraph.packages.count == 1, "\(packageGraph.packages)")
 
             // Find the build tool plugin.
-            let buildToolPlugin = try XCTUnwrap(packageGraph.packages[0].targets.map(\.underlyingTarget).filter{ $0.name == "X" }.first as? PluginTarget)
+            let buildToolPlugin = try XCTUnwrap(packageGraph.packages.first?.targets.map(\.underlyingTarget).filter{ $0.name == "X" }.first as? PluginTarget)
             XCTAssertEqual(buildToolPlugin.name, "X")
             XCTAssertEqual(buildToolPlugin.capability, .buildTool)
 
@@ -1203,7 +1203,7 @@ class PluginInvocationTests: XCTestCase {
             XCTAssertNoDiagnostics(observability.diagnostics)
 
             // Find the build tool plugin.
-            let buildToolPlugin = try XCTUnwrap(packageGraph.packages[0].targets
+            let buildToolPlugin = try XCTUnwrap(packageGraph.packages.first?.targets
                 .map(\.underlyingTarget)
                 .filter { $0.name == "Foo" }
                 .first as? PluginTarget)